1. Field of Invention
The present invention relates to a method of forging high strength alloys, particularly those adapted to gas turbine engine use, in hot die means at elevated temperatures. In particular, an improved die material useful in an apparatus for such a forging process is disclosed.
2. Description of the Prior Art
The problems associated with fabricating high strength alloys, such as those used in gas turbine engines, are well known. A solution to these problems is discussed in the Moore et al patent, U.S. Pat. No. 3,519,503, which has a common assignee with the present invention. By the fabrication method of the patent, high strength alloy billets are processed in compression under controlled conditions of temperature and reduction to place the alloy in a temporary condition of low strength and high ductility and are subsequently forged at a low strain rate in hot dies at a temperature which will maintain the alloy in the aforementioned temporary condition. The forged alloy is then returned to its normal condition of high strength and hardness by conventional heat treatment. An apparatus for use in such a forging process is disclosed in the Moore et al patent, U.S. Pat. No. 3,698,219, which also has a common assignee with the present invention.
During the slow forging step of the process, the alloy must be worked within about 350.degree. F of its normal recrystallization temperature, which may be 1800.degree. F or above in the case of nickel-base superalloys, in order to maintain it in a temporary condition of low strength and high ductility until the desired shape is produced. This necessitates heating the dies to a similar temperature and preferably maintaining that temperature substantially uniformly throughout the dies. As a result, the dies must be made of a material which is capable of withstanding high compressive forces at elevated temperatures and which preferably possesses sufficient thermal conductivity to insure uniform temperature distribution throughout the dies. In the prior art, refractory alloys or superalloys have been found suitable as such die materials. For example, such materials as TZM molybdenum alloy, TRW2278 nickel-base alloy and the nickel-base alloy of U.S. Pat. No. 3,655,462 have been widely used. However, these materials, as well as other similar materials, are extremely expensive and difficult to machine. A non-metallic die material useful in high temperature forging is disclosed in copending application entitled "Diffusion Bonding Separator" and assigned Ser. No. 298,043, now U.S. Pat. No. 3,945,240.
The Jahn patent, U.S. Pat. No. 3,917,884, discloses a method of making carbon-carbon reinforced shapes which may find application as hot pressing molds in fabricating ceramic or refractory metal powders. In hot pressing, the metal powder is placed in a mold and pressed therein at an elevated temperature and pressure so that diffusion bonding of the powder particles occurs. An inherent limitation of hot pressing refractory metal powder in a mold is that only simple shapes, such as cylinders, bars or the like, can be made. Thus, the molds utilized are of simple shape and have no intricate details incorporated therein. On the other hand, in forging high strength alloys in accordance with the Moore et al patents, dies of intricate detail are utilized to produce complex articles, such as gas turbine engine parts. Such dies contain numerous stress raisers, including sharp radii, notches and the like, which magnify the stress exerted on the dies during forging.